In transcutaneous electrical stimulation, the positioning and surface contact of the electrode in relation to the skin of the patient to be stimulated is very important. For optimum results, it is necessary that the electrode be accurately positioned in relation to the muscle and nerve to be stimulated and that good surface contact exists between the electrode and the skin to ensure maximum transmission of electrical stimulation to the nerve. It is widely known that skin tissue, in and of itself, is a relatively poor conductor of electrical stimulation because of its relative dryness. To overcome this problem, it has been known to "wet" the skin with a conductive fluid, such as water or a gel material, to increase electrical transmission therethrough. U.S. Pat. Nos. 4,583,547 and 4,580,572 to Granek et al disclose garments of a non-conductive web material having a plurality of electrodes connectable to a source of electrical stimulation positioned thereon. A space is defined between the web material and each electrode into which a conductive fluid is inserted. The conductive fluid seeps through the small interstices in the garment fabric to "wet" the skin. Stimulation is thus carried through the fabric by means of the conductive fluid.
A problem with such an electrode and garment is that the conductive fluid which wets the skin is not confined to a specific area below the electrode. In the embodiments shown, as the gel penetrates the garment to wet the skin it may also ooze or migrate beyond the outer periphery of the electrode. In this respect, gel migrating beyond the periphery or outer perimeters of the electrode may affect the net charge distribution from the electrical stimulation. In other words, as the conductive fluid migrates beyond the electrode, the area of conductance between the skin and the electrode increases substantially, and for a given current, the current density (i.e. amperage per square inch) decreases. For example, if a 25 milliamp current is delivered to a circular electrode having an area of 8 square inches (the radius of a circle having an area of 8 square inches is approximately 1.6 inches), the current density is approximately 3.125 milliamps per square inch. If the radius of this circle were to increase by 0.4 inches (the result of the oozing or migration of the conductive fluid), the area would increase to 12.6 square inches and the current density would decrease to 1.98 milliamps per square inch. Such as a drop is in the magnitude of approximately 33%. To maintain the same current density to the skin, the stimulator output must be increased by approximately one-third. As can be seen, the inability to maintain the gel within a clearly defined space beneath the electrode greatly reduces the reliability and accuracy of the electrode and the garment.
Another problem with garments of the type shown in the aforementioned patents to Granek et al., is that because the electrodes are relatively small there is little room for error with respect to positioning the electrode on the garment so that it aligns wih the "motor point" on the body of the wearer. The term "motor point" refers to the location on the surface of the skin where our electrical stimulus most easily elicits a muscle contraction. The location of these points on the body is conventionally known throughout the industry, however the spacing between respective points on the body will vary depending upon the size of the given individual. In this respect, garments with small electrodes are generally limited in use to a specific individual, or to individuals with very similar stature. Larger electrodes on garments would provide a greater likelihood of overlaying a "motor point" and therefore are preferable. However, with larger electrodes, should migration of the conductive fluid beyond the periphery of the electrodes occur, a substantial increase in the contact area results with a decrease in the current density.
These and other problems are overcome by the present invention wherein an electrode assembly is provided which confines the conductive fluid within a predetermined area beneath the electrode, and thereby enables more accurate and effective stimulation of the muscle tissue. By being able to accurately control the area wetted by the conductive gel, larger electrodes may be used to ensure operative contact with the wearer's motor points.